(a) Field of the Invention
This invention relates to a system and method for detecting different types of metal objects and more particularly, but not by way of limitation, to a non-obstructive weapon detection system which allows an observer to discriminate between a handgun made of a ferromagnetic material and other objects carried by a person.
(b) Discussion of Prior Art
In the past, gun detection became an important technological requirement in the 1960s with the first occurrences of hijacking of commercial aircraft by armed terrorists. In short order, an electromagnetic gun detection system was developed and deployed at airports around the World. The technology used was basic, with the gun detector being no more than a coil of wire wrapped around a frame through which a passenger desiring to board an aircraft had to pass. The loop was instrumented to measure a quantity, the impedance of the loop, that was monitored on a control panel. If a person carrying a gun were to pass through the portal, the electromagnetic field from the coil would induce currents in the gun. This extraction of energy would appear on a monitor as a change in impedance of the coil and trigger an alarm. The alarm indicating that the individual was carrying some metallic object and should be searched.
The above mentioned metal detection system has been strikingly successful, even though it cannot truthfully be called a gun detection system. The system merely indicates that a person is carrying some small metal article on his person and serves as a cause for that person to be physically searched. The search is the true act of gun detection. The electromagnetic scanning serves only to select the most likely candidates for a physical search. But, this is enough of a deterrent that very few people are found carrying handguns on attempting to board an aircraft. Over the past three decades, the rate of gun detection has been about 1 in 100,000 people and these have almost always been individuals with a legitimate need and who overlook the procedures required to carry a handgun on board the aircraft.
More modern versions of airport gun detection systems are improved over the earlier systems but are not significantly different in their capabilities. But, because they have been very effective, there has not been a need for significant changes in airport gun detection systems. However, there are other arenas in which guns pose an even more dangerous threat to the public welfare. These gun threats being in the commission of armed robbery, in acts of political terrorism and in peacekeeping operations in unstable countries. In these situations, the airport gun detection systems have not been effective.
The limitation to the general use of electromagnetic gun detection is that the cooperation of the public must be complete. People choosing to board an aircraft are as concerned with gun detection as the authorities and so are willing and even anxious to divest themselves for a few moments of the metallic objects they are carrying, queue up for passage through the gun detection portal and then go on about their business. In everyday life, very few people are willing to do this, and the ones who are willing, do not carry handguns. Thus, electromagnetic gun detectors have met with essentially a zero level of acceptance by the public. Further, those business establishments that have employed them, use them only rarely because they don""t wish to discourage customers from entering their business.
Perhaps, wider acceptance of electromagnetic gun detection systems outside an airport setting could be achieved by the development of a system which is discrete. By this, the presence and operation of the system is not obvious to persons being scanned and the system provides a highly reliable discrimination against non-gun metal objects. The subject invention described herein is intended to meet these objectives.
In U.S. Pat. No. 5,552,705 to one of the subject inventors, a non-obtrusive weapon detection system and method for discriminating between a concealed weapon and other metal and non-metal objects is disclosed. In this patent, the detection system describes the use of transmitting low intensity electromagnetic step pulses which cause eddy currents to flow in a metal object under observation. The eddy currents create a plurality of scattered signals which are sufficient to plot a xe2x80x9cdecay curvexe2x80x9d due to the relaxation of the eddy currents. A receiver is used to detect the scattered signals from the eddy currents. Computer equipment is used to digitally process the scattered signals for defining the decay curve. The decay curve provides an accurate determination of the conductive properties of the object under surveillance. This processed information allows an observer to identify the object under surveillance as either threatening or non-threatening.
The subject invention measures both the scattered signals from the eddy currents and a magnetization field signal of the object under observation for an improved weapon detection system. Also the new system and method measures a xe2x80x9ccrossoverxe2x80x9d point at which the magnetization signal and the induction current signal are of the same amplitude but opposite in sign.
Heretofore, there have been a variety of patents describing the use of electromagnetic detection systems in underground mining applications and exploration. In U.S. Pat. Nos. 5,260,660, 5,185,578, 5,066,917 and 4,994,747 by Stolarczykz, apparatus and methods of detecting underground electrically conductive ore bodies and ore zones are disclosed using transmitted electromagnetic energy. Both downhole receivers and downhole transmitters are used in these detection systems.
In U.S. Pat. Nos. 4,821,023 and 4,866,424 to Parks, a current state-of-the-art walk-through metal detector using electromagnetic waves for detecting weapons is described. The metal detector is typical of the detection systems found at airports, court houses and other buildings requiring this type of security system. In U.S. Pat. No. 4,821,023 to Ziolkowski, the use of wave propagation equations is described for producing localized pulses of wave energy. In U.S. Pat. No. 4,978,920 to Mansfield et al., a magnetic screen is developed using a coil surrounded by a set of the electrical conductors. The conductors are placed in a specific region in space.
None of the above mentioned patents specifically describe the a system and method of improved detection of hand guns and other types of weapons made of ferromagnetic material with a low false alarm rate. The improved detection system accomplished by measuring a total electromagnetic field. The total electromagnetic field being the sum of both an incident electromagnetic field and an electromagnetic field scattered from the object.
In view of the foregoing, it is a primary object of the present invention to provide a non-obtrusive weapon detection system which can accurately discriminate between different types of weapons and other objects carried on a person, carried in packaging, luggage, storage containers and other transportation means where different types of weapons may be hidden or smuggled.
Another object of the invention is a person under surveillance need not be aware that he or she is being monitored. The system provides for detection without invading a person""s privacy.
Still another object of the system and method of detection is the increased high probability of detecting a weapon made of ferromagnetic material with a low false alarm rate. The weapon detection system is designed to discriminate between a concealed weapon and other metal and non-metal objects passing through an area where they are illuminated by an electromagnetic field.
Yet another object of the invention is unlike today""s obtrusive metal detectors, a person is not required to turn over metal objects carried on the person to a security guard or to walk through a portal in order to determine the presence of a potentially dangerous metal object. The subject detection system can be used in various applications in public and private buildings, large and small gathering areas along with other types of military and non-military security requirements.
The subject improved weapon detection system includes a magnetic field transmitter which transmits a low intensity electromagnetic signal. The electromagnetic signal illuminates a volume or space wherein the weapon, called a target, may or may not present and carried by a person, in luggage, packaging, etc. The electromagnetic signal is in a form of a sudden steplike change in a constant magnetic field, called a xe2x80x9ctime-domainxe2x80x9d excitation. If a ferromagnetic weapon is detected, a transient waveform is generated by a time-domain excitation. The newly created transient waveform is in the form of two signals. The two signals include a magnetization field signal and a scattered eddy current signal. A receiver is used for detecting the magnetization field signal, the eddy current signal and a crossover point of the step pulse. The two signals and the crossover point are analyzed and digitally processed using a preprogrammed computer. The processed information is then compared with similar values of weapons and non-weapons in a stored data base in the computer to predict the nature of the target. The analyzed information allows an observer to identify the target as being a threatening weapon made of a ferromagnetic material or a non-threatening metal or non-metal object.
These and other objects of the present invention will become apparent to those familiar with electromagnetic detection systems and weapon detection systems as described in the following detailed description, showing novel construction, combination, and elements as herein described, and more particularly defined by the appended claims, it being understood that changes in the precise embodiments to the herein disclosed invention are meant to be included as coming within the scope of the claims, except insofar as they may be precluded by the prior art.